a servos job is to convert electrical power from the receiver to mechanical force....some servos aren't for aircraft use ..as they don't always center in the same spot or "hunt" for center...what servo you need is determined by size of the servo, or speed the servo travels and the torque required.....some servos will not tolerate 6 volts, most will...6 volt over 4.8 is termined by the usage of the servo....6 volts will increase the speed and torque of the servo....
servo can fail, in different ways...gears strip out...one side can open causing a hard over servo...the servo can hunt for center....
Servos are for the most part reliable..but if you have only one on a control surface and it goes bad in flight...you're gonna have your hands full...